This invention relates to an improvement of sealed lead-acid batteries one objective of which is to provide a sealed lead-acid battery having good life performance.
When a conventional open flooded-type cell is used for fork lifts and in other applications where deep discharge is performed cyclically, it is common practice to use tubular positive plates having the positive active material enclosed within a tube made of either glass or synthetic resin fibers. Upon discharge of a positive plate, PbO.sub.2 as the active material is usually converted to PbSO.sub.4 of a larger volume, so that the shape and the volume of the positive plate will change greatly as the number of discharge cycles increases. Hence, if deep discharge is performed cyclically, the binding strength of the positive active material will decrease, causing a gradual decrease in the discharge capacity. Tubular positive plates having the positive active material enclosed within a tube are capable of suppressing the expansion and dislodging of the active material, so they are known to have a long cycle life with the binding strength of the active material being maintained for an extended period. For these reasons, tubular positive plates are also used in the art to manufacture sealed lead-acid batteries having good cycle life performance.
The following three methods are generally known to be applicable for sealing cells:
(1) an electrolyte is impregnated and retained in the plates and separators that are made of fine glass fibers and that conform to the shape of positive and negative plates to fabricate a "retainer type" cell;
(2) an electrolyte is gelled with colloidal silica or the like to fabricate a "gel-type" cell; and
(3) silica granules are packed between plates and around a plate group consisting of the plates and a separator, and an electrolyte is impregnated and retained in the plates, separators and the silica granules to fabricate a "granulated silica retainer type" cell.
Method (1) has not been commercialized since the cost of preparing separators is very high. Compared to flooded-type cells, the cells fabricated by methods (1)-(3) have lower discharge capacities and their life performance is also considerably low. Further, these cells experience greater deterioration in performance than cells employing pasted plates, the inherent advantages of using tubular plates yet being not fully exploited.
In order to investigate the reasons for these problems, the present inventors have conducted various experiments on gel-type and granular type cells and found the following.
In sealed lead-acid batteries of the types described above, most of the electrolyte is retained in the gel or granular silica. However, the transport speed of H.sub.2 SO.sub.4 through those retainers is much slower than in flooded-type cells, so H.sub.2 SO.sub.4 in the electrolyte will be transported into the active material at a much slower speed than in flooded-type cells. As a result, when sealed cells that use tubular positive plates are discharged, H.sub.2 SO.sub.4 is consumed mainly in the neighborhood of a tubular positive plate, and the following phenomena occur within the cells.
In sealed cells that use tubular plates, only a very small amount of electrolyte is present near the contact areas of two adjacent tubes as can be seen from FIG. 2 which shows, in cross section, the essential components of a conventional sealed lead-acid battery including a tubular positive plate 1, posted negative plate 2, separator 3, gelled electrolyte 4, table 5, lead spine 6 and positive active material 7. As a result, the active material at the contact areas of adjacent tubes is supplied with only a small amount of electrolyte, the discharged capacity therefore being reduced. FIG. 3B shows, diagrammatically, PbSO.sub.4 distribution in the cross-sectional positive plate for the case described above. Clearly, PbSO.sub.4 which is a discharge product, is distributed unevenly across the thickness of the positive plate. FIGS. 3A-3C show the profiles of distribution of PbSO.sub.4 after discharging the tubular positive plate. FIG. 3A refers to the cell of the present invention; while FIGS. 3B and 3C refer to conventional sealed and flooded-type cells, respectively.
In the flooded-type cell (FIG. 3C), H.sub.2 SO.sub.4 in the electrolyte moves very quickly. As a result, the areas of contact between tubes are replenished with H.sub.2 SO.sub.4 from other areas as soon as it is consumed, such that the discharge of the positive plate will proceed uniformly in a substantially concentric manner as shown in FIG. 3C.
The above results show that the discharge capacity of the sealed cell is much smaller than in the flooded-type because the transport speed of H.sub.2 SO.sub.4 is slow. Also, the use of tubular plates results in a greater drop in capacity compared to the flooded-type because discharge of tubular positive plate is less likely to occur in areas of contact between tubes. Consequently, in the sealed cell, current density will localized in an area where discharge is highly likely to occur, the active material therein deteriorating prematurely so as to shorten the life of the cell.